The present invention relates to a semiconductor laser device used for an optical pickup of a 3-beam tracking system or the like in an optical disk system, for example.
Conventionally, as a semiconductor laser device used for an optical pickup of 3-beam tracking system in an optical disk system, there has been the device as shown in the plan view of FIG. 4 (refer to Japanese Patent Laid-Open Publication No. SHO 63-175490). In this semiconductor laser device 100, a laser chip 102 is die-bonded onto a submount 101. The submount has a fore end surface 103. In a plane view, an acute angle is formed between a fore end surface 103 and a light-emitting end surface 105 of the laser chip. With this arrangement, light incident on the fore end surface 103 of the submount as indicated by the arrow A is reflected in a direction different from the direction of the light emission axis of the laser chip 102 as indicated by the arrow B.
Light is emitted from the laser chip and condensed on an optical disk by an optical system, and part of the light, a sub-beam in particular, is reflected on this optical disk to return to the semiconductor laser device 100 via the optical system as return light. Then, by the above-mentioned way, the return light is prevented from being reflected in the direction of light emission of the laser chip on the fore end surface of the submount. This prevents reduction of the S/N (signal-to-noise) ratio of a signal read by the optical pickup, which reduction is derived from the fact that the return light reaches the optical disk again via the optical system to be detected by a light-receiving element.
FIG. 5 is a plan view showing another conventional semiconductor laser device (Japanese Patent Laid-Open Publication No. SHO 62-26653). In the plane view of the semiconductor laser device 110, a laser chip 112 is mounted on a submount 111 in such a manner that an acute angle is formed between an edge line of the rectangular submount 111 and an edge line of the rectangular laser chip 112. That is, the acute angle is formed between the fore end surface 113 of the submount and the light-emitting end surface 115 of the laser chip. This arrangement prevents the return light from reaching the optical disk again via the optical system by reflecting the return light incident on the semiconductor laser device 110 as indicated by the arrow C in a direction different from the direction of the light emission axis of the laser chip 112 as indicated by the arrow D.
However, the conventional semiconductor laser device 100 of FIG. 4 has a problem of difficulty in fabrication since it is required to determine the die-bonding position of the laser chip 102 to the submount 101 with high accuracy. This is because the heat release of the laser chip 102 changes when changing projection length 107, the distance from the fore end surface of the submount 101 to the light-emitting side of the laser chip in the light-emitting region 106 of a laser chip as shown in FIG. 6, and thereby changing the characteristics of the operating current value, reliability and so on of the semiconductor laser device change. It is required to form the projection length 107 with high accuracy (tolerance of about 15 μm). In this case, the fore end surface 103 of the submount 101 is inclined relative to the light-emitting end surface 105 of the laser chip 102 in a plane view. Therefore, in order to set the projection length 107 to a prescribed amount, it is required to determine, with high accuracy, a position in the direction of the light emission axis of the laser chip 102 and a position of the laser chip in the direction perpendicular to the light emission axis with respect to the submount 101. This leads to the problem of difficult fabrication. This problem occurs similarly in the semiconductor laser device shown in FIG. 5.
Furthermore, in the case of a semiconductor laser device 120 of a hybrid-type dual-wavelength light source in which two laser chips 122 and 122 are mounted on one submount 121 as shown in FIG. 7, it is required to determine not only the projection lengths 127 and 127 of the laser chips 121 and 121 from the inclined fore end surface 123 of the submount but also the isolation 128 in the direction of the light emission axis between the light-emitting end surfaces 125 and 125 of the laser chips with high accuracy (tolerance of about 10 μm). This makes the fabrication of the semiconductor laser device very difficult.